The only time I had to fight claustrophobia is when I had to crawl under a house to thread a gas line through the floor. The house was built on 2x12s that were six inches off of the ground and were on 2 ft. centers. I essentially had to inch my way back about 25 ft. in a 2' x 1.5' tunnel. I had a friend tie a rope on my ankle to help pull me out if I got stuck.

About half way in I started to really feel claustrophobic. I did what many have already suggested. I closed my eyes, slowed my breathing and calmed myself. I was then fine.